Cryogenic refrigerator

ABSTRACT

A cryogenic refrigerator includes a cylinder, a displacer configured to be moved back and forth in the cylinder by a drive unit, an inlet valve configured to be opened in supplying a refrigerant gas into the cylinder, an exhaust valve configured to be opened in exhausting the refrigerant gas from the cylinder, and an expansion space formed in the cylinder and configured to generate a cooling by expanding the refrigerant gas caused by back and forth movement of the displacer. A moving speed of the displacer in the vicinity of a bottom dead center is set to be faster than the moving speed of the displacer in the vicinity of a top dead center.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based upon and claims the benefit of priorityof Japanese Patent Application No. 2011-212239, filed on Sep. 28, 2011,and Japanese Patent Application No. 2012-118332 filed on May 24, 2012,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cryogenic refrigerator, and morespecifically to a cryogenic refrigerator including a displacer.

2. Description of the Related Art

Conventionally, a Gifford McMahon refrigerator (which is calledhereinafter “GM refrigerator”) is known as a cryogenic refrigeratorincluding a displacer. This GM refrigerator is configured to allow thedisplacer to move back and forth in a cylinder by a drive unit.

Moreover, an expansion space is formed between the cylinder and thedisplacer. By allowing the displacer to move back and forth in thecylinder, a refrigerant gas that is supplied to the expansion space isexpanded, so that a cryogenic cooling is generated.

In general, in this kind of GM refrigerator, a moving speed of one cyclein which the displacer moves back and forth at one stroke in thecylinder is set to be the same as a speed of a simple harmonic motion.

Generally speaking, the displacer is in the vicinity of the bottom deadcenter, and the GM refrigerator performs a process that suctions ahigh-pressure refrigerator gas into the cylinder.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided acryogenic refrigerator including a cylinder, a displacer configured tobe moved back and forth in the cylinder by a drive unit, an inlet valveconfigured to be opened in supplying a refrigerant gas into thecylinder, an exhaust valve configured to be opened in exhausting therefrigerant gas from the cylinder, and an expansion space formed in thecylinder and configured to. generate a cooling by expanding therefrigerant gas caused by back and forth movement of the displacer. Amoving speed of the displacer in the vicinity of a bottom dead center isset to be faster than the moving speed of the displacer in the vicinityof a top dead center.

Additional objects and advantages of the embodiments are set forth inpart in the description which follows, and in part will become obviousfrom the description, or may be learned by practice of the invention.The objects and advantages of the invention will be realized andattained by means of the elements and combinations particularly pointedout in the appended claims. It is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory and are not restrictive of the invention asclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an outline configuration diagram of a GM refrigerator of afirst embodiment of the present invention;

FIG. 2 is an exploded perspective diagram showing an enlargedScotch-yoke mechanism provided at the GM refrigerator of the firstembodiment of the present invention;

FIG. 3 is an enlarged diagram showing a slider frame of the Scotch-yokemechanism;

FIG. 4 is a motion curve diagram of the displacer in the GM refrigeratorof the first embodiment of the present invention;

FIGS. 5A through 5H are diagrams for illustrating operation of theScotch-yoke mechanism provided in the GM refrigerator of the firstembodiment of the present invention;

FIG. 6 is a P-V diagram of the GM refrigerator of the first embodimentof the present invention;

FIG. 7 is a diagram showing an effect of the first embodiment of thepresent invention;

FIG. 8 is an enlarged diagram showing a Scotch-yoke mechanism of amodification of the first embodiment;

FIG. 9 is a motion curve diagram of a displacer in a GM refrigerator ofa modification of the first embodiment;

FIG. 10 is an outline configuration diagram of a GM refrigerator of asecond embodiment of the present invention;

FIG. 11A is a diagram showing a valve timing of the GM refrigerator ofthe second embodiment of the present invention;

FIG. 11B is a motion curve diagram of a displacer in the GM refrigeratorof the second embodiment of the present invention;

FIG. 12 is an outline configuration diagram of a GM refrigerator of amodification of the second embodiment; and

FIG. 13 is an outline configuration diagram of a GM refrigerator of athird embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the cryogenic refrigerator disclosed in Japanese Patent No.2617681, when the moving speed of the displacer is set the same as thesimple harmonic motion in one cycle, a pressure increase of therefrigerant gas in the expansion space is not enough because an inletspeed of the refrigerant gas into the expansion space is slow.Accordingly, there is a concern that not enough cooling can be generatedin generating the cooling, and a cooling efficiency is decreased.

Embodiments of the present invention provide a novel and usefulcryogenic refrigerator solving one or more of the problems discussedabove.

More specifically, embodiments of the present invention provide acryogenic refrigerator that can improve a cooling efficiency.

A description is given below, with reference to drawings of embodimentsof the present invention.

FIG. 1 shows a cryogenic refrigerator of a first embodiment of thepresent invention. In the following description, the cryogenicrefrigerator using a Gifford McMahon cycle (which is hereinafter called“a GM refrigerator”) is taken as an example and described thereof.However, application of the present invention is not limited to the GMrefrigerator, but is possible for various cryogenic refrigerators usinga displacer (e.g., a Solvay refrigerator, a Stirling refrigerator, andthe like).

The GM refrigerator 1 of the present embodiment is a two-stagerefrigerator, which includes a first-stage cylinder 10 and asecond-stage cylinder 20. These first-stage cylinder 10 and second-stagecylinder 20 are formed of a stainless steel with a low thermalconductivity. Moreover, the high-temperature end of the second-stagecylinder 20 is configured to be coupled to the low-temperature end ofthe first-stage cylinder 10.

The second-stage cylinder 20 has a diameter smaller than that of thefirst-stage cylinder 10. A first-stage displacer 11 and a second-stagedisplacer 21 are respectively inserted into the first-stage cylinder 10and the second-stage cylinder 20. The first-stage displacer 11 and thesecond-stage displacer 21 are coupled to each other, and are driven tomove back and forth in an axial direction of the cylinders 10, 20 (i.e.,arrows Z1, Z2 directions in the drawing) by a drive unit 3.

Furthermore, regenerators 12, 22 are respectively provided inside thefirst-stage displacer 11 and the second-stage displacer 21. The insideof the regenerators 12, 22 are respectively filled up with regeneratormaterials 13, 23. In addition, a space 14 is formed at thehigh-temperature end in the first-stage cylinder 10, and a first-stageexpansion chamber 15 is formed at the low-temperature end. Moreover, asecond-stage expansion chamber 25 is formed on the low-temperature sideof the second-stage cylinder 20.

The first-stage displacer 11 and the second-stage displacer 21 includeplural gas passages L1 through L4 to let through a refrigerant gas(e.g., helium gas). The gas passages L1 connects the space 14 to theregenerator 12, and the gas passage L2 connects the regenerator 12 tothe first-stage expansion chamber 15. Furthermore, the gas passage L3connects the first-stage expansion chamber 15 to the regenerator 22, andthe gas passage L4 connects the regenerator 22 to the second-stageexpansion chamber 25.

The space 14 on the high-temperature end side of the fist-stage cylinderis connected to a gas supply system 5. The gas supply system 5 isconfigured to include a gas compressor 6, valves 7, 8, a gas passage 9and the like.

An inlet valve 7 is connected to the inlet port side of the gascompressor 6, and an exhaust valve 8 is connected to the exhaust portside of the compressor 6. When the inlet valve 7 is opened and theexhaust valve 8 is closed, the refrigerant gas is supplied from the gascompressor 6 into the space 14 through the inlet valve 7 and the gaspassage 9. When the inlet valve 7 is closed and the exhaust valve 8 isopened, the refrigerant gas in the space 14 is recovered into the gascompressor 6 through the gas passage 9 and the exhaust valve 8.

The drive unit 3 forces the first-stage and second-stage displacers 11,21 to move back and forth in the first-stage and second-stage cylinders10, 20. The drive unit 3 is constituted of a motor 30 and a Scotch-yokemechanism 32. FIG. 2 shows the enlarged Scotch-yoke mechanism 32. TheScotch-yoke mechanism 32 is roughly constituted of a crank member 34 anda Scotch-yoke 36.

The crank member 34 is fixed to a rotational shaft (which is hereinaftercalled “a motor shaft 31”). The crank member 34 is configured to includea crank pin 34 a provided at a location eccentric to a mounting positionof the motor shaft 31. Hence, when the crank member 34 is mounted on themotor shaft 31, the motor shaft 31 and the crank pin 34 a are eccentricto each other.

In addition, in the Scotch-yoke 36, a slide groove 38 is formed so as toextend in directions perpendicular to moving directions of therespective displacers 11, 21 (i.e., directions shown by arrows X1, X2).Accordingly, the Scotch-yoke 36 is formed in a frame shape.

The slide groove 38 formed into the Scotch-yoke 36 engages with a rollerbearing 35. The roller bearing 35 is configured to be able to roll inthe directions of arrows X1, X2 in the slide groove 38. Here forconvenience of explanation, a description is given below about aspecific configuration of the Scotch-yoke 36 and the slide groove 38.

A crank pin engagement hole 35 a that engages with the crank pin 34 a isformed at the center position of the roller bearing 35. Accordingly,when the motor shaft 31 is rotated in a state of the crankpin 34 aengaged with the roller bearing 35, the crank pin 34 a rotates so as todraw an arc, by which the Scotch-yoke 36 moves back and forth indirections of arrows Z1, Z2. At this time, the roller bearing 35 movesback and forth in the directions of the arrows X1, X2 in the slidegroove 38.

The Scotch-yoke 36 is provided with drive arms 37 that extend out in theupward direction and the downward direction. The lower drive arm 37 ofthe drive arms 37 is coupled to the first-stage displacer 11 as shown inFIG. 1. Therefore, when the Scotch-yoke 36 moves in the Z1, Z2directions by the Scotch-yoke mechanism 32 as discussed above, the drivearms 37 moves upward and downward, by which the first-stage andsecond-stage displacers 11, 21 are moved back and forth in thefirst-stage and second-stage cylinders 10, 20.

A drive of the inlet valve 7 and the exhaust valve 8 is controlled by arotary valve (not shown in the drawing) driven by the motor 30. Therotary valve controls the drive so that open and close of the inletvalve 7 and the exhaust valve 8, and the back and forth motions of therespective displacers 11, 21 have a predetermined phase difference. Thisphase difference causes the refrigerant gas to expand in the first-stageexpansion chamber 15 and the second-stage expansion chamber 25, whichgenerates a cooling.

Next, a description is given about operation of the GM refrigerator 1configured to be discussed above.

The rotary valve opens the exhaust valve 7 of the gas supply system 5just before the first-stage and second-stage displacers 11, 21 reach thebottom dead center. More specifically, in the present embodiment, whenthe first-stage and second-stage displacers 11, 21 reach a 30 degreepoint before the bottom dead center (BDC) by the drive unit 3, the inletvalve 7 is configured to be opened. At this time, the exhaust valve 8maintains a closed state.

This allows a high-pressure refrigerant gas generated in the gascompressor 6 to flow into the regenerator 12 formed in the first-stagedisplacer 11 through the gas passage 9 and the gas passage L1. Therefrigerant gas flowed into the regenerator 12 proceeds, being cooled bya regenerator material 13 in the regenerator 12, and subsequently flowsinto the second-stage expansion chamber 25 through the gas passage L4.

After the inlet valve 7 is opened, the first-stage and second-stagedisplacers 11, 21 reach the bottom dead center that minimizes the volumeof the first-stage and second-stage expansion chambers 15, 25 by beingdriven by the drive unit 3, and the downward (i.e., the arrow Z2direction in the drawing) motion is momentarily stopped (i.e., themoving speed becomes zero).

After that, the first-stage and second-stage displacers 11, 21 start tomove upward (i.e., the arrow Z1 direction in the drawing). This causesthe high-pressure refrigerant gas supplied from the gas compressor 6 issupplied into (suctioned into) the first-stage expansion chamber 15 andthe second-stage expansion chamber 25 through the above-mentioned route.Then, the inlet valve 7 is closed when the first-stage and second-stagedisplacers 11, 21 reach a 121 degree point, and the supply of therefrigerant gas from the gas supply system 5 to the GM refrigerator 1 isstopped.

After the inlet 7 is closed, when the first-stage and the second-stagedisplacers 11, 21 further move upward and reach a 170 degree point, therotary valve opens the exhaust valve 8. On this occasion, the inletvalve 7 maintains the closed state. This causes the refrigerant gases inthe first-stage and second-stage expansion chambers 15, 25 to expand,which generates coolings in respective expansion chambers 15, 25.

After the exhaust valve 8 is opened, the first-stage and second-stagedisplacers 11, 21 reach the top dead center by being driven by the driveunit 3, and stop moving upward (i.e., the arrow Z1 direction in thedrawing), which means the moving speed becomes zero. After that, thefirst-stage and second-stage displacers 11, 21 start to move downward(i.e., the arrow Z2 direction in the drawing). As a result, therefrigerant gas expanded in the second-stage expansion chamber 25 flowsinto the regenerator 22 through the gas passage L4; passes theregenerator 22, cooling the regenerator material 23 in the regenerator22; and flows into the first-stage expansion chamber through the gaspassage L3.

The refrigerant gas flowed into the first-stage expansion chamber 15flows into the regenerator 12 through the gas passage L2. Therefrigerant gas flowed into the regenerator 12 proceeds forward, coolingthe regenerator material 13, and is recovered into the gas compressor 6of the gas supply system 5 through the gas passage L1, the gas passage 9and the exhaust valve 8. Then, the exhaust valve 8 is closed when thefirst-stage and second-stage displacers 11, 21 reach a 340 degree point,and the recovery (suction) treatment of the refrigerant gas from the GMrefrigerator 1 to the gas supply system 5 is stopped.

By repeating the above cycle, a cryogenic temperature of about 20 to 50K or less can be generated in the first-stage expansion chamber 15, anda very low temperature of about 4 to 10 K or less can be generated inthe second-stage expansion chamber 25.

Here, focusing on the Scotch-yoke 36 constituting the drive unit 3, adescription is given about a structure and a function thereof, mainlyreferring to FIGS. 2 and 3.

FIG. 3 is a diagram of the Scotch yoke 36 as seen from the front. Asmentioned above, the slide groove 38 that extends in the X1, X2directions is formed in the Scotch-yoke 36. A conventional slide groovein the Scotch-yoke is formed into a horizontally long rectangular shapein general.

In contrast, the present embodiment is configured to include a convexpart 39 provided at a position corresponding to the bottom dead center(i.e., a position shown by an arrow A in FIG. 3, which is hereinaftercalled the “bottom dead center corresponding position A”) of thedisplacers 11, 21 in the slide groove 38 so as to protrude upward (i.e.,in the Z1 direction). Moreover, a concave part 45 is formed about at aposition corresponding to the top dead center of the displacers 11, 21(i.e., a position shown by an arrow B, which is called hereinafter the“top dead center corresponding position B”) in the slide groove 38 so asto hollow upward (i.e., in the Z1 direction).

Here, a line segment that extends in the vertical direction (i.e., theZ1, Z2 directions) and passes through the bottom dead centercorresponding position A is assumed. This line segment is shown by analternate long and short dashed line in FIG. 3, and is hereinaftercalled a center line Z. The above discussed drive arm 37 is configuredto form a straight line with the center line Z.

The convex part 39 is made of an arc shape centering a position shown byan arrow O in the drawing (which is hereinafter called a center pointO), and is configured to form a circular shape part.

In the present embodiment, the convex part 39 has a symmetric shape inan arrow X1 direction side and an arrow X2 direction side with thecenter line Z at its center.

Accordingly, if a straight line connecting the center point O to the endon the X1 direction side of the convex part 39 is made of a line segmentC, and a straight line connecting the center point O to the end on theX2 direction side of the convex part 39 is made of a line segment D, anangle θ1 between the line segment C and the center line Z is the same asan angle θ2 between the line segment D and the center line Z (θ1=θ2).

A measure of these angles is not specified, but is set at θ1=θ2=30degrees in the present embodiment. However, these angles are not limitedto this, for example, may be set in a range of 20 degrees ≦(θ1=θ2)≦40degrees.

Here, the angles θ1, θ2 that define a formation range of the convex part39 are not necessarily set at the same angle to each other as mentionedabove, but may be configured to have different angles (θ1≠θ2).

Next, a description is given about operation of the respectivedisplacers 11, 21 using the Scotch-yoke mechanism 32 including theScotch-yoke 36 configured as discussed above, with reference to FIGS. 4and 5.

FIG. 4 is a motion curve diagram of the displacers 11, 21. Furthermore,FIGS. 5A through 5H show operations of the roller bearing in the slidegroove 38.

Here in FIG. 4, the transverse axis shows a rotation angle (i.e., crankangle) of the crank member 34, and the longitudinal axis shows adisplacement (travel distance) of the second-stage displacer 21. Inaddition, a characteristic of the GM refrigerator 1 of the presentembodiment is shown by a solid line (which is shown by an arrow A in thedrawing), a characteristic of a conventional GM refrigerator without theconvex part 39 and the concave part 45 is shown by an alternate long andshort dashed line (which is shown by an arrow B).

In the Scotch-yoke mechanism 32 of the present embodiment, the crankangle 0 degree is set at a 30 degree point before the bottom dead center(BDC). Hence, as shown in FIG. 5A, a position of the roller bearing 35in the slide groove 38 when the crank angle is 0 degree is located at aborder between a lower horizontal part 40 and the convex part 39.

When the crank member 34 rotates 30 degrees from this state, followingthis, the roller bearing 35 biases the Scotch-yoke 36 downward (i.e., ina Z2 direction). This operation causes the roller bearing 35 to be movedin an X2 direction in a slide groove 38. More specifically, the rollerbearing 35 engages with the convex part 39 caused by the movement, andenters a state of the roller bearing 35 running on the convex part 39.

As stated above, because the crank pin 34 a to which the roller bearing35 is attached is provided at a position eccentric to the center of thecrank member 34, following the movement of the roller bearing 35, theScotch-yoke 36 moves toward the Z2 direction. In addition, thedisplacers 11, 21 are connected to the Scotch-yoke 36 via the drive arm37. Because of this, as the Scotch-yoke 36 moves, the displacer 11, 21move toward the Z2 direction.

Here, the moving speed of the Scotch-yoke 36 (which is equal to themoving speed of the displacers 11, 21) is noted.

The convex part 39 protrudes compared to the lower horizontal part 40.Hence, a travel distance of the Scotch-yoke 36 per unit time when theroller bearing 35 is engaged with the convex part 39 is longer than whenthe roller bearing 35 is engaged with the conventional horizontal part46 (see FIG. 3).

In other words, the moving speed V1 of the Scotch-yoke 36 movingdownward (in the Z2 direction) following the movement of the rollerbearing 35 (see FIG. 4) becomes faster than the moving speed V1B of theScotch-yoke 36 when the roller bearing 35 is engaged with theconventional lower horizontal part 46 (V1B<V1).

FIG. 5B shows a state of the crank angle being 30 degrees. In thepresent embodiment, the displacers 11, 21 are set to be the bottom deadcenter (BDC) when the crank angle is 30 degrees. Due to this, in thebottom dead center, the roller bearing 35 is located at the top (thecenter position) of the convex part 39.

Following the crank member 34, when the roller bearing 35 passes theposition corresponding to the bottom dead center (BDC) of the displacers11, 21, the moving direction of the Scotch-yoke 36 is reversed. In otherwords, after passing the bottom dead center (BDC), the Scotch-yoke 36starts to move upward (in the Z1 direction).

At this time, the crank angle maintains a state of the roller bearing 35being engaged with the convex part 39 while the crank angle is from thebottom dead center (BDC) to 30 degrees. More specifically, the rollerbearing 35 keeps the state of the roller bearing 35 being engaged withthe convex part 39 (concretely, a part on the X2 direction side relativeto the center axis Z), and moves to a position facing the horizontalparts 40, 41 (the state of which is shown in FIG. 5C).

Accordingly, the moving speed V2 (see FIG. 4) of the Scotch-yoke 36moving upward (in the Z1 direction) caused by the movement of the rollerbearing 35 becomes faster than the moving speed V2B of the Scotch-yoke36 when the roller bearing 35 is engaged with the conventionalhorizontal part 46 (V2B<V2). This is similar to a case where the rollerbearing 35 moves from the state shown by FIG. 5A to the state shown byFIG. 5B.

Moreover, as shown in FIG. 5D, when the crank member 34 further rotates,the roller bearing 35 moves and reaches a position facing the horizontalparts 40, 41 in the slide groove 38. A moving speed V3 of theScotch-yoke 36 in the Z1 direction is made V3 at this time. The movingspeed V3 of the Scotch-yoke 36 is approximately the same as theconventional moving speed V3B because the roller bearing 35 is engagedwith the horizontal part 40.

Furthermore, as stated above, a shape of the convex part 39 issymmetrical about the center line Z in the present embodiment.Accordingly, the moving speeds V1, V2 of the Scotch-yoke 36 in a rangeof back and forth 30 degrees of the bottom dead center correspondingposition A is different in direction but the same in absolute value.Here, when the shape of the convex part 39 is made symmetric about thecenter line Z, production of the Scotch-yoke 36 is made simple.

In addition, as stated above, in the present embodiment, the arc-shapedconvex part 39 is structured to directly engage with the horizontal part40. However, in order to make the roller bearing 35 move smoothly, asmooth connection part (e.g., a straight line) may be provided betweenthe arc-shaped convex part 39 and the horizontal part 40.

FIGS. 5E through 5H show operation of the roller bearing 35 when engagedwith the concave part 45. The concave part 45 is made of a hollow shaperelative to the upper horizontal part 41. With respect to this concavepart 45, while the roller bearing 35 is engaged with the concave part45, a moving speed V4 of the Scotch-yoke 36 (i.e., displacers 11, 21) isslower than a moving speed V4B of the Scotch-yoke 36 when the rollerbearing 35 is engaged with the conventional horizontal part 47 (V4<V4B).

Moreover, the concave part 45 is formed across a range of ±30 degreeswhen expressed in a crank angle of the crank member 34, centering aposition to be the top dead center corresponding position B.Accordingly, as shown in FIG. 4, the moving speed V4 of the displacers11, 21 in the range of ±30 degrees with the top dead center (TDC) at thecenter is slower than the moving speed V4B of the Scotch-yoke 36 whenthe roller bearing 35 is engaged with the conventional horizontal part47 (V4<V4B).

Then, the crank member 34 further rotates from the state shown in FIG.5G, as shown in FIG. 5H, the roller bearing 35 moves to a positionfacing the horizontal parts 40, 41 in the slide groove 38. This causesthe Scotch-yoke 36 to start moving, which further causes the displacers11, 21 to start moving.

If a moving speed of the Scotch-yoke 36 in the Z1 direction at this timeis made V5, because the roller bearing 35 is engaged with the horizontalpart 41, the moving speed V5 is approximately the same as theconventional moving speed V5B.

As is clear from the above description, the GM refrigerator 1 of thepresent embodiment is set so that the moving speeds V1, V2 at the bottomdead center of the displacers 11, 21 are faster than the moving speed V4at the top dead center (V4<V1, V4<V2). Therefore, as shown in FIG. 4,the motion curve of the displacers of the present embodiment (a solidline shown by an arrow A in the drawing) has a steeper characteristiccurve than the motion curve of the displacers of the conventional GMrefrigerator (an alternate long and short dashed line shown by an arrowB) in the vicinity of the bottom dead center.

Here, “the moving speeds of the displacers 11, 21 at the bottom deadcenter” mean moving speeds of the displacers 11, 21 in a range of theconvex part 39 formed in the slide groove 38. Moreover, “the movingspeeds at the top dead center” means moving speeds of the displacers 11,21 in a range of the concave part 45 formed in the slide groove 38.

Furthermore, the GM refrigerator 1 in the present embodiment isconfigured to allow the inlet valve 7 to be opened when the displacers11, 21 reach the point of 30 degrees before the bottom dead center (BDC). Hence, in the present embodiment, when the inlet valve 7 is opened,the moving speed of the displacers 11, 21 changes from V5 to V1 (whichis faster than the conventional V1B) at the same time.

Here in the present embodiment, a timing when the moving speeds of thedisplacers 11, 21 (Scotch-yoke 36) change in the vicinity of the topdead center is set to be the same as a timing when the inlet valve 7 isopened, but the timing of the inlet valve 7 being opened can be setearlier than the timing when the moving speeds of the displacers 11, 21(Scotch-yoke 36) change.

In a case of the above-mentioned configuration, since the inlet valve 7is opened until the displacers 11, 21 reach the bottom dead center(Scotch-yoke 36) , the moving speeds of the displacers 11, 21(Scotch-yoke 36) become fast.

In addition, in the present embodiment, since the displacers 11, 21(Scotch-yoke 36) reach the bottom dead center until the exhaust valve 8is opened, the moving speeds of the displacers 11, 21 (Scotch-yoke 36)become approximate the same as the moving speeds of the conventionaldisplacers. More specifically, the moving speeds of the displacers 11,21 (Scotch-yoke 36) are changed from the moving speed V2 to the movingspeed V3 at 30 degrees of the crank angle, and become approximately thesame as the conventional moving speed V3B. Here, the inlet valve 7 inthe present embodiment is closed at 121 degrees of the clank angle.

Next, a description is given about a functional effect of setting themoving speeds V1, V2 at the bottom dead center of the displacers 11, 21faster than the moving speed V4 at the top dead center.

As discussed above, by allowing the inlet valve 7 to be opened, thehigh-pressure refrigerant gas is supplied from the gas supply system tothe GM refrigerator 1. The refrigerant gas has a characteristic whosedensity increases as pressure increases. Hence, pressure loss becomessmall as pressure increases.

In addition, in the present embodiment, by increasing the moving speedsV1, V2 of the displacers 11, 21 at the bottom dead center, a gas flowrate from the gas supply system 5 into the GM refrigerator 1 can beincreased. In this manner, even if the gas flow rate into the GMrefrigerator 1 is increased, the pressure loss is low because therefrigerant gas is at a high pressure.

This enables a large amount of refrigerant gas to be supplied into theGM refrigerator 1 efficiently.

Therefore, after supplying the refrigerant gas to the GM refrigerator 1,and opening the exhaust valve 8 after closing the inlet valve 7, it ispossible that a large amount of the refrigerant gas can be expanded.Therefore, cooling efficiency of the GM refrigerator 1 can be improved.

In this way, in order to supply the high-pressure refrigerant gas to theGM refrigerator 1, it is favorable to configure the GM refrigerator 1 soas to increase the moving speeds of the displacers 11, 21 since theinlet valve 7 is opened until the displacers 11, 21 reach the bottomdead center.

FIG. 6 shows a P-V line diagram of the GM refrigerator 1 in the presentembodiment (a characteristic shown by an arrow A), and a P-V linediagram of a GM refrigerator without the convex part 39 in the slidegroove 38 (a characteristic shown by an arrow B) as a comparativeexample together.

In the P-V line diagram, a cooling capacity generated in one cycle ofthe GM refrigerator corresponds to an area surrounded by the P-Vdiagram. Referring to FIG. 6, it is noted that the area of the P-Vdiagram of the present embodiment is larger than that of the P-V diagramof the conventional GM refrigerator. Accordingly, FIG. 6 demonstratesthat the GM refrigerator 1 of the present embodiment has a highercooling efficiency than that of the conventional GM refrigerator.

FIG. 7 is a table showing a cooling temperature of the GM refrigerator 1of the present embodiment, compared with a cooling temperature of theconventional GM refrigerator. In both GM refrigerators, a temperaturenear the first-stage expansion chamber and a temperature near thesecond-stage expansion chamber are measured.

As shown in FIG. 7, a first-stage temperature of the GM refrigerator ofthe present embodiment was 45.1 K in comparison with that of theconventional GM refrigerator being 46.2 K. Moreover, a second-stagetemperature was 4.19 K compared to that of the conventional GMrefrigerator being 4.26 K. Therefore, FIG. 7 also demonstrates that theGM refrigerator 1 of the present embodiment has a higher coolingefficiency than that of the conventional GM refrigerator.

FIG. 8 shows a Scotch-yoke mechanism 48 of a GM refrigerator that is amodification of the present embodiment. More specifically, FIG. 8 showsan enlarged Scotch-yoke 49 of the Scotch-yoke mechanism 48. In FIG. 8,the same numerals are put to components corresponding to those shown inFIG. 1 through FIG. 5, and the description is omitted.

The Scotch-yoke mechanism 32 provided in the GM refrigerator 1 shown inFIG. 1 through FIG. 5 is configured to provide the concave part 45 inthe upper horizontal part 41 of the Scotch-yoke 36. In contrast, thepresent modification features not to provide the concave part 45 in theupper horizontal part 41 but to be configured to be flat.

FIG. 9 is a motion curve diagram of displacers 11, 21 of the GMrefrigerator using the Scotch-yoke 49 shown in FIG. 8. In the GMrefrigerator of the present modification, because the concave part 45 isnot provided in the upper horizontal part 41, the displacers 11, 21 arenot stopped in the vicinity of the top dead center, and the movementbecomes like a simple harmonic motion.

Here, moving speed of the displacers 11, 21 from a point of 30 degreesbefore the bottom dead center (TDC) to the bottom dead center is V4 a,and moving speeds of the displacers 11, 21 from the top dead center to apoint of 30 degrees after the top dead center (TDC) is V4 b.

As discussed above, because the roller bearing 35 is engaged with theconvex part 39 formed in the lower horizontal part 40, the moving speedsV1, V2 of the displacers 11, 21 in the vicinity of the bottom deadcenter (BDC) are faster than those when the roller bearing 35 is engagedwith the horizontal parts 40, 41. Accordingly, the configuration of themodification also allows the moving speeds V1, V2 of the displacers 11,21 in the vicinity of the bottom dead center to be faster than themoving speeds V4 a, V4 b of the displacers 11, 21 in the vicinity of thetop dead center.

Therefore, with the modified GM refrigerator, the cooling efficiency canalso be improved like the above-mentioned GM refrigerator 1 of theembodiment.

Here in the present embodiment, a description is given about an exampleof the convex part 39 being an arc shape, but a shape of the convex part39 is not limited to the arc shape. As long as the convex part 39 has ashape protruding above the lower horizontal part 40, for example,configuring the convex part 39 by combining plural straight lines andcurves is possible.

Next, a description is given about a second embodiment of the presentinvention.

FIG. 10 shows a GM refrigerator 50 of the second embodiment. In thepresent embodiment, a description is given about a one-stage GMrefrigerator as an example.

The GM refrigerator 50 includes a drive unit 51, a displacer 52, acylinder 54, a cooling stage 55, a regenerator 57, compressor 62 and thelike. The GM refrigerator of the present embodiment features to adopt apneumatic mechanism as the drive unit 51 to drive the displacer 52.

The displacer 52 is configured to include a displacer body 52A, a lowertemperature side thermal conduction part 52B, regenerator 57 and thelike. The displacer body 52A is formed into a cylindrical shape withcaps on the end, and the regenerator 57 housing a regenerator materialis provided therein.

A rectifier 59 that rectifies a flow of a refrigerant gas is provided onthe high-temperature side (the upper side is the high-temperature sidein the drawing). Furthermore, a rectifier 60 that rectifies a flow of arefrigerant gas is also provided on the low-temperature side (the lowerside is the low temperature side in the drawing).

In a top plate part 52D that is located at the high-temperature end ofthe displacer 52, plural flow passages 61 are provided to flow therefrigerant gas from a room temperature chamber 58 to the regenerator57. The room temperature chamber 58 is formed between the top plate part52D of the displacer 52 and a top plate part 54A of the cylinder 54.

This room temperature chamber 58 is connected to the compressor 62. Morespecifically, the room temperature chamber 58 is connected to a supplypipe 67 that is connected to a supply side of the compressor 62, and isconnected to a return pipe 68 that is connected to a return side of thecompressor 62. The supply pipe 67 is connected to the room temperaturechamber 58 through an inlet valve 63 (which may be called V1). Moreover,the return pipe 68 is connected to an exhaust valve 64 (which may becalled V2) through the room temperature chamber 58. Here, the respectivepipes 67, 68 join together and become one on the downstream side of therespective valves 63, 64 and are connected to the room temperaturechamber 58.

Therefore, when the inlet valve 63 is opened and the exhaust valve 64 isclosed, the high-pressure refrigerant gas generated by the compressor 62is supplied to the room temperature chamber 58. In contrast, when theinlet valve 63 is closed and the exhaust valve 64 is opened, therefrigerant gas is flowed back from the room temperature chamber 58 tothe compressor 62.

On the low-temperature end of the displacer 52, a low-temperature sidethermal conduction part 52B is provided. Furthermore, a second passage66 in communication with the regenerator 57 and an expansion space 53 isformed between the displacer 52A and the low-temperature side thermalconduction part 52B. The low-temperature side thermal conduction part52B is joined to the displacer body 52A by using a pin 56.

The expansion space 53 is formed between the cylinder 54 and thedisplacer 52 (the low-temperature side thermal conduction part 52B). Thehigh-pressure refrigerant gas from the compressor 62 is introduced intothe expansion space 53. In addition, the expansion space 53 isconfigured to generate a cooling therein by allowing the introducedrefrigerant gas to be adiabatically expanded.

The cylinder 54 houses the displacer 52 in a movable state therein. Thecylinder 54 has a cylindrical shape with caps on the ends, and a coolingstage 55 is provided on the low-temperature end to be an opening side.This cooling stage 55 is thermally connected to an object to be cooled,and the object to be cooled is cooled by a cooling generated in theexpansion space 53.

Moreover, a seal 65 is installed between the cylinder 54 and thedisplacer 52. This seal 65 prevents the refrigerant gas supplied fromthe compressor 62 from passing a gap between the displacer 52 and thecylinder 54 and from flowing into the expansion space 53.

On the high-temperature side of the cylinder 54, a drive unit 51 thatdrives the displacer 52 is provided. The drive unit 51 is configured toinclude a drive piston 52E, a drive chamber 70, a high-pressure drivingvalve 71, a low-pressure driving valve 72, and the like. Furthermore, inthe present embodiment, a high-pressure refrigerant gas generated in thecompressor 62 is used as a driving gas.

The drive piston 52E constitutes a wall on the displacer side of thedrive chamber 70, and is configured to be integrated with the displacer52. The drive piston 52E can be provided, for example, so as to protrudeupward from the center position of the top plate 52D of the displacer52. Accordingly, when the drive piston 52E moves up and down, followingthis, the displacer 52 moves up and down in the cylinder 54.

The drive chamber 70 is formed at the center position of the top placepart 54A of the cylinder 54. This drive chamber 70 is configured toprotrude upward from the top plate 54A, and the above drive piston 52Eis configured to be movable in a vertical direction (in an axialdirection of the cylinder 54) in the drive chamber 70.

In addition, a seal 73 is installed at a predetermined position of thedrive chamber 70. The seal 73 is installed between an inner wall of thedrive chamber 70 and the displacer 52E. This allows the drive chamber 70to be configured to be hermetically separated from the room temperaturechamber 58. Moreover, by providing the seal 73, the drive piston 52E canmove up and down, maintaining a hermetical state of the drive chamber70.

Furthermore, the drive chamber 70 is connected to the compressor 62.More specifically, the supply pipe 67 and the return pipe 68 areconnected to the drive chamber 70. The supply pipe 67 is connected tothe drive chamber 70 through the high-pressure driving valve 71 (whichmay be called the valve V3) . Also, the return pipe 68 is connected tothe drive chamber 70 through the low-pressure driving valve (which maybe called the valve V4). Here, the respective pipes 67, 68 join togetherand become one on the downstream side of the drive chamber 70, and areconnected to the drive chamber 70.

Therefore, when the high-pressure driving valve 71 is opened and thelow-pressure driving valve 72 is closed, the high-pressure refrigerantgas generated in the compressor 62 is supplied to the drive chamber 70,and a pressure (which is hereinafter called a “P2”) in the drive chamber70 becomes high. In contrast, when the high-pressure driving valve 71 isclosed and the low-pressure driving valve 72 is opened, the refrigerantgas in the drive chamber 70 flows back to the compressor 62 and thepressure P2 in the drive chamber 70 becomes low.

In this manner, the pressure P2 in the drive chamber 70 can becontrolled by opening and closing the high-pressure driving valve 71 andthe low-pressure driving valve 72. On the other hand, a pressure (whichis hereinafter called a “P1”) in the cylinder 54 can be controlled byopen and close of the inlet valve 63 and the exhaust valve 64.

Accordingly, when the pressure P1 in the cylinder 54 becomes higher thanthe pressure P2 in the drive chamber 70 (P2<P1) by the open and closecontrol of the respective valves 63, 64, 71 and 72, the displacer 52moves upward (the movement toward the top dead center direction). On thecontrary, when the pressure P1 in the cylinder 54 is lower than thepressure P2 in the drive chamber 70 (P1<P2), the displacer 52 movesdownward (the movement toward the bottom dead center direction). In thisway, the GM refrigerator 50 of the present embodiment is configured tocause the drive unit 51 to drive the displacer 52.

Here, the respective valves 63, 64, 71 and 72 (valves V1, V2, V3, andV4) are configured to be integrated as a rotary valve, and the GMrefrigerator 50 is configured to cause the displacer 52 to move back andforth once (i.e., to perform one cycle movement) by one revolution ofthe rotary valve (revolution of 360 degrees).

Next, referring to FIG. 11, a description is given about operation ofthe GM refrigerator 50 configured as the above.

FIGS. 11A and 11B show operation of the GM refrigerator 50 of thepresent embodiment. FIG. 11A shows valve timing of the GM refrigerator50 of the present embodiment, and FIG. 11B shows movement of thedisplacer 52 in the GM refrigerator 50.

Here in FIG. 11A, bold solid lines show periods that respective valves63, 64, 71 and 72 (valves V1, V2, V3 and V4) are opened, and atransverse axis shows a rotation angle of the rotary valve (which ishereinafter just called “a valve rotation angle”) . Moreover, in FIG.11B, a transverse axis shows a rotation angle of the rotary valve, and alongitudinal shows an amount of displacement of the displacer 52.

Referring to FIG. 11A, when the valve rotation angle is 0 degree, onlythe high-pressure driving valve 71 (V3) constituting the drive unit 51is opened, and the other valves 72, 63 and 64 (V1, V2 and V4) are keptclosed. Hence, the refrigerant gas whose pressure is raised by thecompressor 62 is supplied to the drive chamber 70 through thehigh-pressure driving valve 71 (V3).

Because of this, the pressure P2 in the drive chamber 70 becomes higherthan the pressure P1 in the cylinder 54 (P1<P2). Accordingly, thedisplacer 52 moves downward toward the bottom dead center (BDC). Here, amoving speed of the displacer 52 in moving downward is made a VC1.

In the GM refrigerator 50 of the present embodiment, the bottom deadcenter (BDC) is set at an earlier angle (lower angle) than the valverotation angle 90 degrees. In addition, the exhaust valve 63 (v1) is setto be opened at an earlier valve rotation angle θ1 than the bottom deadcenter (BDC). Moreover, the high-pressure driving valve 71 (V3) is setto be closed at a valve rotation angle θ2 between the valve rotationangle θ1 and the bottom dead center (BDC).

In this manner, when the high-pressure driving valve 71 (V3) is closedand the inlet valve 63 (V1) is opened, the high-pressure refrigerant gasis introduced from the compressor 62 into the cylinder 54 (the roomtemperature chamber 58 and the expansion space 53) though the inletvalve 63 (V1). This causes the pressure P1 in the cylinder 54 to beincreased.

Furthermore, when the displacer 52 reaches the bottom dead center (BDC),the low-pressure driving valve 72 (V4) is opened. Due to this, since thedrive chamber 70 is connected to the return pipe 68, the internalpressure P2 becomes low. Accordingly, the pressure P1 in the cylinder 54becomes higher than the pressure P2 in the drive chamber 70, and thedisplacer 52 moves upward toward the top dead center (TDC). Here, amoving speed of the displacer 52 in moving upward is made a VC2.

Following this upward movement of the displacer 52, the high-pressurerefrigerant gas generated by the compressor 62 is flowed into theexpansion space 53 through the room temperature chamber 58, the flowpassage 61, the regenerator 57, and the second flow passage 66.

On this occasion, the refrigerant gas is cooled by the regeneratormaterial in the regenerator 57.

The inlet valve 63 (V1) is closed at a valve rotation angle θ3. At thetime of this valve rotation angle θ3, the cylinder 54 is filled up withthe high-pressure refrigerant gas, and the internal pressure P1 is kepthigh. In addition, the low-pressure driving valve 72 (V4) is kept openat the valve rotation angle θ3, and the pressure P2 in the drive chamber70 is kept low. Because of this, the displacer 52 continues to moveupward even at the valve rotation angle θ3.

In the present embodiment, the GM refrigerator 50 is configured to openthe exhaust valve 64 (V2) at the valve rotation angle 180 degrees. Whenthe exhaust valve 64 (V2) is opened, the refrigerant gas in theexpansion space 53 expands, and thereby a cooling occurs in theexpansion space 53. The cooling generated in the expansion space 53cools the object to be cooled connected to the cooling stage 55.

As stated above, after the exhaust valve 64 (V2) is opened, thelow-pressure driving valve 72 (V4) is kept opened. By allowing theexhaust valve 64 (V2) to be opened, the pressure P2 in the cylinder 54is low. Similarly, by allowing the low-pressure driving valve 72 (V4) tobe opened, the pressure P2 in the drive chamber 70 is low. Moreover, inthis state, space parts (the expansion space 53, the room temperaturechamber 58 and the like) formed in the cylinder 54 and the drive chamber70 are both connected to the return pipe 68.

Accordingly, in a state of the exhaust valve 64 (V2) and thelow-pressure driving valve 72 (V4) being both opened, the pressure P1 inthe cylinder 54 and the pressure P2 in the drive chamber 70 areapproximately the same (P1≈P2). Thus, in a state of the pressure P1 inthe cylinder 54 and the pressure P2 in the drive chamber 70 beingapproximately the same, the displacer 52 keeps being approximatelystopped. Hence, a speed of the displacer 52 at this time is made a VC3,where VC3 equals approximately 0.

The low-pressure driving valve 72 (V4) is closed at a valve rotationangle θ4. Furthermore, when the low-pressure driving valve 72 (V4) isclosed, the high-pressure driving valve 71 (V3) is opened at a valverotation angle θ5 after that.

The low-pressure driving valve 72 (V4) is closed and the high-pressuredriving valve 71 (V3) is opened, by which the high-pressure refrigerantgas is flowed into the drive chamber 70 from the compressor 62, and thepressure P2 in the drive chamber 70 is increased. On the other hand, theexhaust valve 64 (V2) is kept opened even at the valve rotation angleθ5, and the pressure P1 in the cylinder 4 is kept low. Accordingly, byallowing the high-pressure driving valve 71 (V3) to be opened, the drivepiston 52E is biased downward, and the displacer 52 start to movedownward toward the bottom dead center. A speed of the displacer 52 atthis time is made a VC4.

When the displacer 52 moves downward in a state of the exhaust valve 64(V2) being opened, the refrigerant gas in the cylinder 54 such as theexpansion space 53, the room temperature chamber 58 and the like isflowed back to the compressor 62 through the return pipe 68.

After that, the exhaust valve 64 (V2) is closed at a valve rotationangle θ6. This causes the high-pressure refrigerant gas from thecompressor 62 to be supplied only to the drive chamber 70 through thehigh-pressure driving valve 71 (V3), and therefore the downward movingspeed of the displacer 52 becomes the above-mentioned VC1.

As is clear from the above description, in the GM refrigerator 50 of thepresent embodiment, the moving speeds VC1, VC2 of the displacer 52 inthe vicinity of the bottom dead center (BDC) become faster than themoving speed VC3 in the vicinity of the bottom dead center (TDC)(VC3<VC1, VC3<VC2).

This depends on an open period of the low-pressure driving valve 72 (V4)being set to be longer than that of the high-pressure driving valve 71(V3) in the present embodiment. More specifically, the low-pressuredriving valve 72 (V4) is opened while the valve rotation angle is fromthe BDC to θ4 (about 245 degrees) the high-pressure driving valve 71(V3), compared to the high-pressure driving valve 71 (V3) opened whilethe valve rotation angle is from θ5 to θ2 (about 120 degrees) in onecycle (360 degrees).

In this way, by extending the period when the low-pressure driving valve72 (V4) is opened to be longer than the period when the high-pressuredriving valve 71 (V3) is opened, the period when the low-pressuredriving valve 72 (V4) and the inlet valve 63 (V1) are both opened (BDCto θ3), and the period when the low-pressure driving valve 72 (V4) andthe exhaust valve 64 (V2) are both opened (180 degrees to θ4) can beincreased.

In other words, by extending the period (BDC to θ3) when bothlow-pressure driving valve 72 (V4) and inlet valve 63 (V1) are opened,the speed VC2 of the displacer 52 moving upward can be increased. Inaddition, by extending the period (180 degrees to θ4) when bothlow-pressure driving valve 72 (V4) and exhaust valve 64 (V2) are opened,the speed of the displacer 52 becomes slow, and the speed in thevicinity of the bottom dead center (BDC) becomes relatively fast.

Thus, in the present embodiment, since the moving speeds V1, V2 at thebottom dead center (BDC) of the displacer 52 can be increased as well asthe first embodiment, a large amount of refrigerant gas can be suppliedinto the GM refrigerator 50 (expansion space 53) efficiently. Hence, thelarge amount of refrigerant gas can be expanded in the expansion space53 when a cooling is generated, and the cooling efficiency of the GMrefrigerator 50 can be improved.

FIG. 12 shows a GM refrigerator 80 of a modification of the GMrefrigerator 50 of the second embodiment.

Here, in FIG. 12, the same numerals are put to components correspondingto those of the GM refrigerator of the second embodiment, and thedescription is omitted.

The GM refrigerator 80 of the present modification features to include aflow passage resistance valve 81 to be a flow resistance providedbetween the high-pressure driving valve 71 of the supply pipe 67 and thedrive chamber 70. A needle valve that can adjust a valve openingposition can be used for this flow passage resistance valve 81. Byproviding the flow passage resistance valve 81, a flow passageresistance between the high-pressure driving valve 71 of the supply pipe67 and the drive chamber 70 becomes higher.

By doing this, at the time of intake that returns the refrigerant gasfrom the cylinder 54 to the compressor 62, a differential pressurebetween the pressure P1 in the cylinder 54 and the pressure P2 in thedrive chamber 70 can be reduced, and a moving speed of the displacer 52in the vicinity of the top dead center (TDC) can made be further slower.Moreover, by providing the flow passage resistance 81, since a speed ofintroducing a gas into the drive chamber 70 becomes slower, a periodwhen the pressure in the drive chamber 70 is raised up to a highpressure becomes longer. This enables the pressure P1 in the cylinder 54to be higher than the pressure P2 in the drive chamber 70 when the valveV1 is opened, and the moving speed of the displacer 52 in the vicinityof the bottom dead center (BDC) can be further faster. Therefore, thecooling efficiency can be enhanced further.

Here, a component used for the flow resistance is not limited to theneedle valve, but using another component such as an orifice and thelike is possible.

FIG. 13 shows a GM refrigerator 90 of a third embodiment of the presentinvention.

Here in FIG. 13, the same numerals are put to components correspondingto those of the GM refrigerator 50 of the second embodiment, and thedescription is omitted.

The GM refrigerator 90 of the present embodiment features a linear motoras a drive unit 91. This drive unit 91 includes a magnet 92, a driveinductor 93, a control unit 94 and the like.

The magnet 92 is a bar-shaped magnet in which north poles and southpoles are magnetized alternately at a predetermined pitch. This magnet92 is provided at the center part of the top plate part 52D of thedisplacer 52 so as to protrude upward.

The drive inductor 93 is constituted of plural electromagnets. Therespective electromagnets generate magnetic forces by flowing currentstherein. The magnet 92 is inserted into a space formed in the centerpart of the drive inductor 93 movable in a vertical direction. The driveinductor 93 is connected to the control unit 94.

The control unit 94 is to drive and control the drive inductor 93. Morespecifically, the control unit 94 changes a magnitude and a direction ofa current supplying to the drive inductor 93. As discussed above, themagnet 92 is magnetized by the north poles and the south polesalternately at the predetermined pitch. Accordingly, by allowing thecontrol unit 94 to control the magnet poles of the plural electromagnetsconstituting the drive inductor 93 so as to change subsequently, themagnet 92 moves linearly.

The magnet 92 is fixed to the displacer 52. Hence, by causing the driveinductor 93 to move the magnet 92, the displacer 52 is also moved.Accordingly, the displacer 52 can be driven by the drive unit 91.

The movement of the displacer 52 by the drive unit 91 can be adjusted bycontrolling the magnitude and direction of the current flowing throughthe drive inductor 93. In the present embodiment, a micro computer isincorporated in the control unit 94, and a program that is set to causethe displacer 52 to be moved as shown by the solid lines in FIG. 4 isalso incorporated.

Therefore, by allowing the control unit 94 to control the drive inductor93, the displacer 52 is moved as shown by the solid lines in FIG. 4. Bydoing this, because the moving speeds V1, V2 at the bottom dead center(BDC) of the displacer 52 can be increased similarly to the firstembodiment in the present embodiment, a lot of refrigerant gas can besupplied into the GM refrigerator (cylinders 4) efficiently.Accordingly, the lot of refrigerant gas can be expanded in the expansionspace 53 in generating a cooling, and the cooling efficiency of the GMrefrigerator 90 can be enhanced.

In this manner, according to a cryogenic refrigerator of embodiments ofthe present invention, a cooling efficiency can be improved because arefrigerant gas can be efficiently supplied into a cylinder in supplyingthe refrigerant gas.

All examples recited herein are intended for pedagogical purposes to aidthe reader in understanding the invention and the concepts contributedby the inventor to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions,nor does the organization of such examples in the specification relateto a showing of the superiority or inferiority of the invention.Although the embodiments of the present invention have been described indetail, it should be understood that the various changes, substitutions,and alterations could be made hereto without departing from the spiritand scope of the invention.

What is claimed is:
 1. A cryogenic refrigerator comprising: a cylinder;a displacer configured to be moved back and forth in the cylinder by adrive unit; an inlet valve configured to be opened in supplying arefrigerant gas into the cylinder; an exhaust valve configured to beopened in exhausting the refrigerant gas from the cylinder; and anexpansion space formed in the cylinder and configured to generate acooling by expanding the refrigerant gas caused by back and forthmovement of the displacer, wherein a moving speed of the displacer inthe vicinity of a bottom dead center is set to be faster than the movingspeed of the displacer in the vicinity of a top dead center.
 2. Thecryogenic refrigerator as claimed in claim 1, wherein the moving speedof the displacer is increased since the inlet valve is opened until thedisplacer reaches the bottom dead center.
 3. The cryogenic refrigeratoras claimed in claim 1, wherein a part of the movement of the displacercentering the bottom dead center is symmetric.
 4. The cryogenicrefrigerator as claimed in claim 1, wherein the drive unit furtherincludes a Scotch-yoke mechanism, the Scotch-yoke including a bearingand a Scotch-yoke, the Scotch-yoke including a slide groove with whichthe bearing engages movably, and a convex part is provided at a positioncorresponding to the bottom dead center of the slide groove.
 5. Thecryogenic refrigerator as claimed in claim 4, wherein the convex partincludes a circular shape part at the center thereof.
 6. The cryogenicrefrigerator as claimed in claim 5, wherein a straight line part isprovided on both sides of the circular shape part.
 7. The cryogenicrefrigerator as claimed in claim 1, the drive unit further includes adrive chamber to allow a driver gas for driving the displacer, ahigh-pressure valve for supplying the driver gas to the drive chamber,and a low-pressure valve for exhausting the driver gas in the drivechamber.
 8. The cryogenic refrigerator as claimed in claim 7, wherein anopened period of the low-pressure valve is set to be longer than that ofthe high-pressure valve.
 9. The cryogenic refrigerator as claimed inclaim 8, wherein a flow passage resistance is provided between thehigh-pressure valve and the drive chamber.
 10. The cryogenicrefrigerator as claimed in claim 1, wherein the drive unit is a linearmotor connected to the displacer.